1. Field of the Invention
The present invention relates to an ink, an ink cartridge, and an ink jet recording method.
2. Description of the Related Art
Ink jet recording methods using plain paper as a record medium have also been utilized in printing of business text and other documents that include characters, tables, and figures, at a rapidly increasing frequency in such applications. In such applications an ink in which pigment is used as a coloring material (pigment ink) is often used because high levels of color development and fastness of images (resistance to light, ozone gas, water, etc.) are required.
A factor in the superior color development of images recorded with pigment ink compared to that with an ink in which dye is used as a coloring material is a large amount of coloring material on the surface of the record medium. This is because dye permeates deep into a record medium, whereas pigment quickly aggregates because of the evaporation of liquid components during or after the application of the ink to a record medium. Pigment ink, however, causes a low highlighter resistance of images because the pigment as a coloring material often remains on the surface of a record medium.
It has been attempted to add urethane resin to ink to improve the characteristics, for example, of images recorded with pigment ink (refer to PCT Japanese Translation Patent Publication No. 2005-515289 and Japanese Patent Laid-Open No. 9-291242). The ink described in PCT Japanese Translation Patent Publication No. 2005-515289 contains a urethane resin having an acidic group. The ink described in Japanese Patent Laid-Open No. 9-291242 contains a urethane resin that has a specified molar ratio of urethane bonds/urea bonds. This publication also discloses a urethane resin that has no urea bonds.
After studying these inks containing urethane resin afresh, the inventors found that the use of urethane resin admittedly improved the highlighter resistance of images, but not to an extent where the level of resistance demanded in recent years could be achieved.
When a known ink containing urethane resin is ejected through an ink jet recording head, furthermore, the ejection stability and ejection accuracy of the ink are low and need to be improved. The urethane resin described in PCT Japanese Translation Patent Publication No. 2005-515289 was found to give the ink only insufficient ejection stability because of its low acid value of approximately 30 mg KOH/g.
Japanese Patent Laid-Open No. 9-291242 also discloses that the molar ratio of urethane bonds/urea bonds of the urethane resin is in the range of 80.0/20.0 to 50.0/50.0. Ejecting an ink containing such a urethane resin through a recording head results in the urethane resin adhering to the surface of the recording head where the head has an ejection orifice (ejection orifice surface). The urethane resin adhering to the ejection orifice surface catches the subsequent ink droplets, making the ink droplets deviate from the intended direction of ejection. This phenomenon is hereinafter referred to as “irregular ejection.”
Irregular ejection occurs whatever the mode of ejection of the ink. The deviation of ink droplets from the intended direction of ejection is minor immediately after the ink is ejected, but becomes significant by the time when the record medium is reached. Once irregular ejection occurs, ink dots no longer adhere to the intended positions on the record medium, leaving non-recorded areas in streaks on the image and seriously affecting the quality of the image.